The present application relates to the color imaging arts. It finds particular application to color correction tables as used in, e.g., printers, and will be described with particular reference thereto.
Heretofore, computers and other electronic equipment such as digital cameras have typically generated three-dimensional RGB (red, green, blue) color signals. Many printers, however, are configured to receive four-dimensional CMYK (cyan, magenta, yellow, and black) signals as input and, therefore, print CMYK colors which are determined from corresponding RGB values. A lookup table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before being received by the printer. However, because printers inherently have a complex, nonlinear behavior and therefore have a complex nonlinear colorimetric response, even after a printer is calibrated, the full spectrum of CMYK values and printed colors is not a completely accurate representation of the original RGB spectrum. A discrepancy arises because the relationship between digital values that drive the printer and the resulting calorimetric response is a complex nonlinear function. A response or other value labeled as “colorimetric” indicates that the response has been measured by an instrument such as a spectrophotometer. To deal with this problem, a color correction lookup table is built which approximates the mapping between RGB colorimetric space and CMYK values. In other words, the color correction lookup table corrects for nonlinearities and unwarranted absorptions of inks or dyes such that the printer prints the true corresponding color.
The lookup table is a three-dimensional table wherein the RGB coordinates identify a three-dimensional vector location within the three-dimensional space. Each RGB coordinate is typically represented by an 8 bit red value, an 8 bit green value, and an 8 bit blue value. In this case, the RGB coordinate is capable of addressing 2563 locations. However, a table comprising such a large number of locations places a strain on computational resources. For this reason, it is desirable to generate a smaller table which requires less computational power to generate and use, and which is much more compact, which offers added benefit if the table is embedded as part of a document. A color correction table can be as small as 16×16×16 (4096) table locations, each of which stores a four-dimensional CMYK value. However, due to the above-described nonlinear response of the printer, there are areas of the color correction with high curvature that are under-sampled in the table and other smooth areas of the correction that are over sampled unnecessarily. Currently, small color correction tables result in loss of detail and higher delta E (ΔE) accuracy in the darker areas. The term ΔE as used herein refers to a measure of color difference, e.g., a difference between a sample color and a reference color in L*a*b* color space. Heretofore, if the image quality loss was too large, higher density correction tables, i.e., larger tables, were employed to reduce the image quality loss, however, with the above-described associated drawbacks.
The present application provides a new and improved method and apparatus for determining a color correction table which overcomes at least the above-described problems.